The invention relates to multicarrier transmission equipment and more particularly to a receiver adapted to this type of transmission and which is protected from unwanted or interfering signals.
During the reception of signals formed from several modulated carriers, one of the problems to be solved relates to the protection against the effects of unwanted signals. In the reception of signals formed from several modulated carriers, it is not an economic proposition to use one reception channel for each carrier frequency. Thus, the receiver generally comprises high frequency and intermediate frequency circuits common to all the modulated carriers and a separate circuit permitting demodulation for each carrier.
The intermediate frequency amplifier common to all these carriers is an automatic gain control (AGC amplifier), whose gain is controlled by a voltage proportional to the envelope of the signal received having the highest level among the the signals transmitted on the different carriers. Thus, under normal conditions, this arrangement makes it possible to obtain a linear operation of the AGC amplifier.
At the output of this amplifier, the signal is branched to the different channels tuned to the frequencies to be demodulated. A control circuit receiving the voltages detected on the different channels permanently selects the highest detected voltage in order to apply it to the gain control input of the intermediate frequency amplifier.
In the presence of an unwanted signal, the protection of such a receiver is provided by successive high frequency and intermediate frequency filtering operations prior to amplification and by narrow band filtering centered on each of the intermediate frequencies. When the unwanted signal has a frequency within the pass band of the narrow band filters, the interference is at a maximum. Moreover, when the maximum detected voltage is that corresponding to the channel, whose carrier frequency is close to the frequency of the unwanted signal, it is this maximum detected voltage which is used as a reference for the gain control of the intermediate frequency amplifier. Thus, for an increasing level of the unwanted signal, the gain of the intermediate frequency amplifier decreases, so that the level of the signal before demodulation decreases until it reaches the limit value acceptable for a satisfactory operation of the demodulators. The level of the maximum admissible unwanted signal is determined as a function of the level of the useful signals, which must be maintained above the lower threshold of the dynamics of the demodulator.
Such a receiver is able to resist unwanted or interfering signals, whose frequency is close to one of the frequencies received, and whose maximum admissible level is approximately 20 decibels above the highest level useful signal, bearing in mind that the dynamics of the demodulator has a lower threshold of approximately -30 dB compared with the nominal level.